Mujahid (may Allah have mercy on him) narrates from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that a woman of Quraysh made her servant mudabbar, then she wanted to make a kitabah contract (for his freedom) with him, so she wrote to Sayyiduna Abu Hurayrah (may Allah be pleased with him). Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: "Make the kitabah contract with him; if he pays off his kitabah, then that is that, and if she dies, he will be free." And I think he said: "Nothing (of his kitabah) would be hers."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21580